As a result of increasing complexity and miniaturization associated with the electronic assembly and computer arts, the demand for more sophisticated and reliable connectors has increased. Smaller size, lighter weight packaging and an augmented necessity for reliability have virtually rendered obsolete individually soldered connectors in many areas of the industry. For example, printed circuit boards, digital watches, portable calculators, etc., have generated the need for connectors having the ability to reliably connect a large number of electrically conductive traces on closer centers in a compact area. By no means exhaustive, the following list defines certain desirable characteristics for a connector: low contact resistance; close contact spacing; vibration damping; providing an environmental seal; elimination of the need for precise alignment; inherent low contact insertion force; easily modified shape and size to meet specifications; low production cost, etc.
A new class of connectors has evolved to satisfy these characteristics. They are layered elastomeric connectors (LEC). LEC's generally are composed of alternating layers of dielectric elastomer and an elastomer filled or doped with electrically conductive material such as silver particles, graphite particles, conductive fabrics, wires, etc. The dielectric elastomer layers are sandwiched between the conductive layers and are of sufficient thickness to insulate the conductive layers from one another and therefore prevent the formation of electrically conductive or leakage pathways between the conductive layers. Among the many elastomers available for use, silicon rubbers have been settled upon as providing the material properties suitable for manufacture of LEC's.
Silicone rubbers possess a low hardness, are very temperature stable, have a low compression set and reasonable chemical inertness, and lastly, are fairly easy to process and fabricate. An LEC composed of alternating layers of a dielectric silicone rubber and a conductive particle filled silicone elastomer provides a connector having a large number of conductive pathways in a small volume for closer contact spacing which may even provide for redundant contacts for the same electrical traces. Due to the inherent vibration damping of the elastomer, an LEC connecting fragile components will demonstrate protective characteristics especially against fretting corrosion and abrasion. Additionally, the compressiblity of an LEC, upon compression onto electronic traces, provides an environmental seal in the contact zones reducing harmful effects of dust and moisture.
LEC's provide the additional advantage of easy modification of both geometric configuration and size to meet specifications for a particular use. This adaptability as well as the low cost of and ease of manufacture of LEC's, among the other aforementioned features, have generated an increasing demand for LEC's.
An example of an LEC providing these features is described in British Patent No. 2087655. Therein are disclosed a number of embodiments for an LEC having an irregular cross section which includes a series of whisker-like projections from two oppositely facing peripheral surfaces. The projections are composed of discrete unidirectionally aligned linear carbon fiber or metal wire bodies imbedded in an electrically conductive particle-filled elastomer. A further illustration of an LEC is provided in U.S. Pat. No. 4,295,700 disclosing an LEC where the alternating electrically conductive layers may be composed of conductive particle filled elastomer, woven cloth where the woof fibers are conductive but the warp fibers are dielectric or an identical embodiment that was described in the aforementioned British Patent. Although providing many advantages, certain limitations as to the applicability of LEC's do exist. First, the contact resistance of a typical particle filled elastomer is fairly high. The contact resistance results from the transmission of electrical current across the interface between the traces and the connector and vice versa. Where contact resistance measures about 30 ohm/cm, the applicability of LEC's is reduced for interconnecting high impedance, low amperage devices, e.g., liquid crystal devices. Moreover, heat generated by the high contact resistance may raise the temperature of the connector area sufficiently to damage either the connector itself or the electrical elements nearby.
The aforementioned patents contain modified LEC structures employing projections and fibers to reduce contact resistance. However, the use of the fibers or projecting bodies limit the geometrical configuration and applicability of these LEC's by restricting connections along the surfaces from which the fibers or bodies project.
A further disadvantage of known LEC's is cost. An estimate of conventional selling prices for an LEC is approximately one dollar per inch. Where connectors are used in abundance, particularly with the advent of flat display panels, this cost is too high. For example, an 8-inch by 4-inch panel requires nearly two linear feet of connectors. Based on a projected sales price of two hundred dollars, there would be a twenty-four dollar material cost associated with the connectors alone; simply too much for the product. Thus, it is desirable from an economic perspective to develop a lower priced LEC.